Current “hands-free” zoom optics are switchable zoom optics capable of providing 120 degree and 12 degree Fields-of-View (hereafter FOV) while providing scene apparent brightness of 50% in normal vision mode and greater than 25% in 10× zoom mode relative to normal daytime vision. Conventional optics designs imply that the large angle optics must both provide adequate aperture for the 120 degree FOV into a 3.5 mm eye pupil and an effective collection are of a 35 mm diameter entrance pupil at 12 degree FOV. The challenges for these requirements are considered in U.S. Pat. No. 7,123,421.
FIG. 1 shows an example prior art design having a greater than 90 degree FOV configuration. This design provides approximately one-half the desired entrance pupil diameter at the smaller FOV. This constraint implies that the lens elements would need to be doubled in diameter and thickness to support the brightness requirement for the 12 degree FOV.
Wide field zoom 10× lenses (e.g., 120 degrees or larger) are typically extraordinarily large and heavy, particularly when they are required to maintain the apparent brightness of the naked eye. As an example, a 10× zoom requires 100× the photon collection area of the human eye to maintain the same brightness. In addition, maintaining the apparent brightness of the naked eye at 10× zoom usually requires at least 100× the open aperture. As these instances get worse, most optical systems usually have to increase the f# with larger apertures to overcome distortion.
In some existing systems, color correcting a 12 degree FOV at 10× magnification, filling the entire human 120 degree FOV typically requires a very complicated stack of lenses. As the input aperture gets larger, distortion produces a more and more severe limit, generally forcing operation at a stopped down f#, which requires an even larger aperture to maintain collection efficiency. In some existing systems, when you need 1× and 10×, eyepiece vignetting limits off-axis capability.
FIG. 2 illustrates another example prior art optical zoom configuration that includes a flat-optic where the entrance pupil is circumferential. The illustrated flat-optic might be adapted to 10× magnification; however, it requires powered surfaces in the internal reflections.
As an example, some optical systems have historically often included a first inward-facing surface that is at least partly reflective and a second inward-facing surface that is at least partly reflective. These reflective surfaces are known to reflect received light toward an aperture. US2010/0188856A1 utilized this known type of technology to development a flat-optic.
One of the drawbacks with the flat-optic is that it cannot reasonably be adapted to multiple levels of zoom. Another drawback with the flat-optic is that it typically requires significant manufacturing start up expense.